This invention relates to an alignment apparatus, usable in manufacture or observation of semiconductor devices such as integrated circuits, large scaled integrated circuits and very large scaled integrated circuits, for positioning a wafer with respect to an imaging plane of an optical system. More particularly, the invention relates to an alignment system, usable in a projection exposure apparatus, for positioning a wafer relative to the image of a mask.
A recent tendency to reduction in the minimum width of line in a circuit pattern constituting a semiconductor circuit device requires a higher resolving power even in a projection exposure apparatus. In order to achieve the higher resolving power, the position of each of a mask and a wafer must be accurately located at a focal point position of an imaging optical system of the projection exposure apparatus.
Such positioning or alignment in the exposure apparatuses has been performed by, for example, abutting an upper surface of a wafer, which has been flattened by means of a super-flat plate (wafer chuck) acting on the wafer from behind, against pawls formed at three points on a reference surface (wafer disk) located at a predetermined position. According to this method, however, an adhesive resist material applied to the wafer surface is liable to adhere to the pawls on the reference surface (wafer disk). The amount of resist adhesion increases with the increase in the number of the wafers treated. This obstructs accurate positioning of the wafer relative to the reference surface and results in disadvantageous defocus. Moreover, the areas of the wafer which abut against the pawls are not exposed to the image light of the photomask. This leads to a decreased yield of the semiconductor devices.
In order to overcome such problems, it has been proposed to detect the positional relation between the wafer and the imaging plane of the optical system in a non-contact manner such as optically, acoustically or with the use of air, and to adjust the position of the wafer relative to the imaging plane on the basis of the results of detection. U.S. Pat. No. 4,344,160 issued Aug. 10, 1982 discloses a system of such. In the system disclosed in this patent, there are provided a plurality of distance detectors and a plurality of drive sources, corresponding respectively to the distance detectors, for moving the surface of an object, i.e. the wafer, in the direction of an optical axis to thereby position the wafer relative to the imaging plane. This position adjustment is also effective to flatten the wafer surface.
However, this system involves inconveniences such as follows:
In this system, all the position detectors are opposed to one, the same flat plate and, in accordance with a detection signal supplied from each detector, an associated one of the drive sources is controlled. This means that the actuation of one of the drive sources affects the detection information of each of the other detectors. In other words, when one of the drive sources is actuated, in response to the detection information supplied from an associated one of the detectors, to move the surface of the object in the direction of optical axis to a predetermined point, the positional information of the other detectors varies. It is therefore difficult to achieve high speed and high precision positioning by means of these combinations of the detectors with the drive sources. In some cases, the positioning movement will not be terminated and oscillation will occur. In these cases, it is necessary to examine the interrelation between each of the drive sources and a corresponding one of the detectors to effect matrix control on the basis of high speed computation of the examined interrelation. This processing is not easy and requires a complicated control system. Further, if the detectors are spaced apart from the driving sources or if the positional relation with respect to the horizontal direction is variable, the position control becomes more and more difficult. For these reasons, the system disclosed in the aforementioned U.S. Patent can not easily be incorporated into a projection exposure apparatus having a mechanism in which a drive source is movable horizontally relative to a detector.